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A&A 520, A10 (2010)based on the thermometry and the ground calibration will beused to provide a relative calibration of the sub-millimeterchannels on the scale of a week. Future balloon experimentsobserving the galactic structure in the submillimeter range onangular scales comparable to HFI Planck (e.g., PILOT) willalso provide valuable information about this relative calibration.Finally, the absolute calibration of FIRAS will be usedto obtain the required 3% accuracy.2.8. Detection noiseAlthough noise in the detectors and in the readout electronicscan potentially have a significant impact on the ultimate sensitivityof the measurements, the goal of the HFI is to reach theultimate physical limitation which is the intrinsic statistical fluctuationsin the astrophysical background and foreground photons.The detector noise was measured during all phases of theinstrument development starting with the electrical NEP of blindbolometers, and continuing with an optical background equivalentto the expected flight background on the integrated horn,filter, and bolometer subsystems.Thesemeasurementswerealsoreproduced during the focal plane calibration. The detector noisemeasurements were carried out across a wide range of frequencies,from a few mHz to the cut-off frequency of the low passfilter needed by the analog-to-digital conversion of the lock-indetection (a few kHz). The DPU (digital processing unit) canprovide the full sampling of any pixel before demodulation. ThisDPU operation mode was used to monitor and measure the noiselevel at audio frequencies during the FPU ground calibrationand in-flight. However the most important measurements willcome from in-flight data obtained in the demodulated (nominal)mode of the DPU. The ground-based measurements will be usedto understand these data and to extract the different contributionsto this noise including background photon noise, intrinsicdetector noise, voltage and current noise in the electronics,and the temperature stability of both the optical components andthe detectors.2.9. CrosstalkDuring the focal plane calibration, the electrical crosstalk waschecked by applying a bias signal toeachbolometerinturnandmonitoring the response of the neighboring devices. To simulatethe stray capacitance of the wiring on the spacecraft, a harnessidentical to the flight harness was used for the ground test setup.On the ground, optical crosstalk and spectral leaks were checkedat the FPU level using illuminators in front of a subset of thepixels. In-flight limits to the crosstalk amplitude will be deducedfrom the sky data by applying two methods:– scans across bright point sources, including planets;– measurements of the cross-correlation between channels.2.10. Readout electronicsThe characteristics of the readout electronics are measured atboth the subsystem level (without the bolometers) and the instrumentlevel. The bolometers are bias modulated at a frequencyclose in value to 180 Hz. The bias frequency is derived fromaclockcommontothe4Kcoolercompressorswhichisoperatedexactly at 1/4.5 times this frequency, or about 40 Hz. The4Kcooleristheonlymechanicalsubsystemoperatedabovethe spin frequency of the satellite (1 rpm). The stability of thebias generator, the pre-amplifier and amplifier gains, and thereadout electronic noise are continuously monitored during theflight. The bias control parameters were optimised both duringthe focal plane calibration and in-flight for the actual instrumentand telescope background prior to the initiation of the first survey.Absolute in-flight calibration requires that the response ofthe instrument be stable or at least well known for periods ofat least 15 days (see Sect. 2.7). The absolute calibration of themeasurement chain does not need to be superior to a few percent,although knowledge of the relative response must be accurateto a fraction of a percent, with a goal of 0.2% accuracy forthe relative time response and deviation from linearity. A similaraccuracy is required for the change of response with the background(static load on bolometers) and bolometer plate temperature.These accuracies are achieved by performing dedicatedcalibration tests, both on the ground and during the flight, andby using sophisticated models of the detection chain.2.11. CompatibilityAll compatibility issues are checked on the ground. Tests arecarried out at the FPU level and during the tests of the integratedsatellite: compatibility with the cryocoolers, with the LFI,and with the service module subsystems. The susceptibility ofthe detectors and electronics to high energy particles hitting wasmonitored during the focal plane calibration. Based on the prelaunchknowledge of the particle environment at L2 and dependingon the energy threshold, the rate of particle hits at L2 is expectedto be between 1 and 10 hits per minute. For these rates,and based on the amplitude of the particle hits observed on thesignal during the calibrations, masking the data to below thelevel of the noise removes a negligible fraction of the integrationtime. However, the true rate and the corresponding energywill be known only in orbit.3. Focal plane calibration in the Saturne cryostatThe focal plane of HFI was calibrated in the Saturne cryostat ofthe calibration facility of the Institut d’Astrophysique Spatialein Orsay. The HFI focal plane consists of the detectors, the opticalfilters and horns, the 100 mK stage of the dilution cooler,and the thermo-mechanical structure and its interfaces with the4Kand18Kcryocoolers.Wedesignedandbuiltanopticalandcryogenic simulator dedicated to this calibration.3.1. Description of the Saturne simulator for HFI3.1.1. Optical layoutThe main difficulty in performing calibration measurements ofthe HFI is simulating a radiative background environment that isrepresentative of the L2 orbit. An additional complication is therepeatable simulation of the very low amplitude signal expectedfrom the astrophysical sources (Fig. 2). To achieve these requirements,the Saturne cryostat gives a thermal environment consistingof a 2 K enclosure that contains the HFI FPU and the opticalcalibration system. The calibrator is an integrating sphere fed byboth internal sources and a path to external sources. A sphericalmirror couples the output of the calibrating sphere to theFPU. Finally, an instrumented wheel can be deployed in front ofthe FPU, producing polarization and optical crosstalk measurements(Figs. 3 and 4). The 2 K enclosure (Fig. 5) isopticallysealed, and the optical bench is partially covered by ThomasKeating microwave absorber so that the cavity behaves like a2KblackbodysourcewhenallothersourcesareeitherturnedPage 4 of 15
F. Pajot et al.: Planck pre-launch status: HFI ground calibration10 -11Power on detector (W)10 -1210 -1310 -1410 -15HFI10 -160.2 51Wavelength (mm)Total expected in orbitAstrophysicalTelescope4 K stage optics2 K blackbody in front of HFI4 K blackbody in front of HFIFig. 2. Background radiation absorbed by the single mode detectors(λ 2 etendue) with a ∆λ = λ/4 bandwidth.Thetelescopeemissionis simulated by a 60 K Planck spectrum with an emissivityof 0.005(λ/1 mm) −1/2 .Fig. 4. Optical setup on the 2 K Saturne cryoplate with the HFI PFM.The polariser and sources on the instrumented wheel are positioned outof the optical path to the integrating sphere.80K shield20K shield2K shieldexternal sourcespointsourcemirrorpolarisercrosstalksourcesHFIFTSfilter wheelintegrating sphereblackbodiesreferencebolometer2K plateFig. 3. Optical diagram of the HFI FPU calibration setup in the Saturnecryostat.Fig. 5. Optical setup after integration of the 2 K optical baffle. The HFIdetectors are looking into the 2 K enclosure. Only the back of the HFIPFM is visible, with its 4 K and 18 K interfaces.off or blocked. The 2 K cavity is itself enclosed by vapor-cooled20 K and 80 K shields. The scattering of the light inside thesphere is obtained by a pseudo random machining of its innersurface. Two internal thermal sources and two external sources(an external chopped source and a Fourier transform spectrometer)feed the integrating sphere.3.1.2. The Saturne cryostatThe Saturne cryostat (Fig. 6) wasusedtoperformtheISOCAMcalibrations (Vigroux et al. 1993). However, we completed animportant redesign to meet the requirements for the ground calibrationof the HFI instrument. The Saturne cryostat consists of avacuum chamber (1600 mm high, 1604 mm diameter) attachedto the ground by three legs. All the utilities and all the hermeticinterfaces are located at the bottom of the vacuum chamber. TheHFI calibration requires 24 h periods of operation that are uninterruptedby cryogen transfers from the 4 K storage tank to the2Kchamber.Thelowesttemperatureachievedis1.8K.3.1.3. HFI thermal interfacesThe HFI FPU provides only the 100 mK dilution cooler stage.The Saturne cryostat provides the cooling interfaces simulating:– the 50 K stage for the cold JFET preamplifier box (the JFETthemselves are heated to 130 K);– the 18 K sorption cooler;– the 4 K Joule-Thomson cooler;– the LFI.3.1.4. Internal optical sourcesThe integrating sphere is fed by two low-power thermal sources:1. CS1 is an emissive ring heated to a temperature adjustable to30 K, with an effective etendue of 1000 mm 2 sr, located insidethe integrating sphere. This black body produces a backgroundsimilar to that expected from the Planck telescope inflight conditions.Page 5 of 15
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A&A 520, E1 (2010)DOI: 10.1051/0004
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ABSTRACTThe European Space Agency
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A&A 520, A1 (2010)57 Instituto de A
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-30-40-30-6-3-20A&A 520, A7 (2010)0
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